It is worth it, as others have said, just for the stable, automated build process itself. But just having it and in use could begin to change some internal practices you didn't even know you wanted to change in the first place (not saying that's your situation, just that it's possible).
Case in point, I recently managed a group that, over the course of the year, went from nearly the same type of environment as you listed to one that incrementally used more features of CI (Jenkins, specifically) and to their benefit. For instance, the group switched from a mix of SVN and git to a mix of public and private GitHub repos with post-commit hooks back to Jenkins, allowing them to better distribute and manage their different types of projects. Additionally, test coverage improved -- included going from none to some, and from some to much more -- including from some folks who just didn't like writing tests, because seeing everyone else's tests break or not made them feel like they had to keep up and/or join the fun (this was a neat thing to watch, don't know how common it is!). Deployment moved from batch scripts and manual processes to push-button deployment of builds, with an eye toward never doing it the "old way" again.
Like most tools, it'll only be as useful as you make it, but even if you use it for the most minimal reasons, I'm a Jenkins fan.
Edited to add: Apropos of nothing, today is Jenkins's birthday.
